Conventionally, a conductive material obtained by using a carbon substance such as a carbon fiber, a carbon nanotube or graphene as well as an electric device such as a photoelectric conversion device obtained by using the conductive material, such as a liquid crystal display, a solar cell, an organic EL device or an optical sensor, have been developed.
In the case of the conductive material obtained by using the carbon substance, rare metal or the like is not used or the amount used of the rare metal can be significantly reduced. Attention has been paid to the conductive material since it has a high flexibility and a strong mechanical strength and is chemically stable.
However, although the conductive material obtained by using the carbon substance has a relatively high conductivity, the resistance in conduction between molecules is large. In the case where the conductive material is used as a transparent electrode with a large area, the electric resistance becomes higher in the same light transmission compared to an indium tin oxide (ITO) film. In the case where the conductive material is used as a long-distance wire or the like, the electric resistance is still higher compared with a metal conductive material such as copper (Cu).
Accordingly, the carbon substance is complexed with metal or semiconductor particles or wires to improve the conductivity.
Electric devices such as a liquid crystal display, a solar cell, an organic EL device have two electrodes and a functional layer sandwiched therebetween. As the transparent electrode, the indium tin oxide (ITO) film is generally used.
As the negative electrode of photoelectric conversion devices such as a solar cell or an organic EL device, aluminum (Al), which has a small work function, or a magnesium (Mg) alloy, which has a smaller work function than Al, is used. A photoelectric conversion device is disclosed, which uses the ITO film as a negative electrode and metal having a large work function as a positive electrode.
As an inexpensive, stable, and flexible transparent electrode obtained without using indium (In) which is the rare metal to be used for the ITO film, a carbon nanotube having an unsubstituted graphene structure or a planar graphene thin film has been examined.